1. Field of the Invention
The present invention relates to a light emitting device having a pattern structure, and more particularly, to a light emitting device having a multiple pattern structure which can easily control defect density and stress distribution of the light emitting device and can increase light extraction efficiency by forming a multiple corrugated pattern on a surface of a substrate used in the light emitting device.
2. Description of the Related Art
Conventional semiconductor light emitting devices can be classified into laser diodes (“LDs”) and light emitting diodes (“LEDs”). LEDs use characteristics of compound semiconductors which convert electrical energy into infrared rays, visible light or signals in the formed of light.
Group III nitride group compound semiconductors are direct transition type semiconductors, and are widely used in light emitting devices such as LEDs or LDs, since stable operation thereof can be obtained at a higher temperature compared to devices that use other semiconductors. Group III nitride group compound semiconductors are usually formed on substrates formed of sapphire Al2O3.
FIG. 1 is a cross-sectional view illustrating a conventional structure of a Group III nitride group compound semiconductor light emitting device. Referring to FIG. 1, an n-GaN layer 12 is formed on a sapphire substrate 11, and an active layer 13, a p-GaN layer 14 and a p-type electrode 15 are sequentially formed on a portion of the n-GaN layer 12. An n-type electrode layer 16 is formed on the portion of the n-GaN layer 12 on which the active layer 13 is not formed.
In a conventional light emitting device as illustrated in FIG. 1, efficiency of light extraction from an inner side of the active layer 13, e.g., a lower side of the active layer 13 facing and in contact with the n-GaN layer 12, to the outside is an important issue. In order to efficiently extract light generated in a longitudinal direction of the sapphire substrate 11 and the active layer 13, efforts to form a transparent electrode or a reflection electrode have been made. However, a large portion of light generated in the active layer 13 proceeds in a horizontal direction (e.g., substantially perpendicular to the longitudinal direction). To extract light in the longitudinal direction, efforts have been made, for example, to form a reflection surface on a sidewall of a stacking structure of a semiconductor device after the sidewall having a predetermined angle is formed. However, this process is difficult and costly. Also, in order to increase optical output of the Group III nitride group compound semiconductor light emitting device which uses the sapphire substrate 11, a flip chip type device structure is employed. However, the light extraction efficiency remains approximately at 40% due to a refractive index difference between GaN and the sapphire substrate 11.
To supplement the light extraction efficiency, an LED structure as depicted in FIG. 2A has been introduced, that is, after a corrugated structure is formed on a surface of a sapphire substrate 21, semiconductor crystal layers including an active layer 22 are formed on the sapphire substrate 21. In the LED structure in FIG. 2A, a corrugated refractive interface is formed on a lower side of the active layer 22 to be able to extract a portion of light in the horizontal direction which normally disappears in the light emitting device.
Also, when a Group III nitride group compound semiconductor is formed on the sapphire substrate 21, a potential is generated due to mismatched lattice constants between the sapphire substrate 21 and the Group III nitride group compound semiconductor. FIGS. 2B (a) through 2B (d) are cross-sectional views illustrating a conventional process of forming a light emitting device on a substrate having a corrugated structure. To prevent mismatched lattice constants from occurring (e.g., a lattice constant miss fit), referring to FIG. 2B-d, the corrugated structure is formed on the sapphire substrate 21, and a GaN layer 23 is formed on the sapphire substrate 21. A process of forming an LED on the sapphire substrate 21 having the corrugated structure will now be described.
To form the GaN layer 23 on the sapphire substrate 21 having a corrugated structure as depicted in FIG. 2B (a), GaN facets 24 are grown on the upper part of patterns of the corrugated structure and sidewalls of each pattern of the corrugated structure, as illustrated in FIG. 2B (b). Afterward, as depicted in FIG. 2B (c), a planarized GaN layer 23 can be obtained. An active layer 22, a GaN layer 25 and a first electrode layer 26 are formed on a first region of the planarized GaN layer 23, and a second electrode layer 27 is formed on a second region of the planarized GaN layer 23. The completed LED is illustrated in FIG. 2B (d).
In the case of a semiconductor light emitting device having a simple corrugated structure, optical extraction efficiency is increased compared to a semiconductor light emitting device which uses a conventional flat substrate. However, there is a need to develop a semiconductor light emitting device which can further increase the optical extraction efficiency.